1. Field of the Invention
This invention relates generally to a new and unique electrochemical machining process, and more specifically to a unique electrochemical machining process which has greatly improved shaping capabilities over comparable prior art techniques by including a relative contacting movement, orbital or otherwise, between the tool and workpiece in one plane, and further effecting a reciprocating motion as a result of the orbital motion or by a separate reciprocal motion in a second plane. This new and inventive process greatly expands the capabilities of electrochemical machining to make possible the electrochemical machining of complex external or internal shapes with a high degree of accuracy and precision not obtainable by prior art electrochemical machining practices. Of particular significance is the fact that the process of this invention provides a relatively simple method of machining such complex shapes as turbine blades with a high degree of accuracy, either individually, or even a whole circle of turbine blades on a complex turbine wheel simultaneously.
2. The Prior art
Electrochemical machining (ECM), is one of the non-traditional machining processes, which is in essence an application of Faraday's Law in a machining process to selectively and controllably remove metal from a workpiece. While the most familiar application of Faraday's Law is in electroplating, where metal is deposited onto a workpiece surface, ECM can be thought of as the reverse of metal plating where metal is removed electrochemically from the workpiece. Because metal removal is electrochemical rather than mechanical or thermal, ECM is ideally suited to the machining of metals which are difficult to machine by the more traditional machining processes, and will not induce any residual stresses or distortion in the machined workpiece. Even heat treated and work hardened metals can be machined by ECM without any adverse affects on the metal's pre-machining properties.
Unlike conventional machining, the ECM tool does not contact the workpiece, and accordingly, removes metal from a workpiece electrochemically atom by atom with virtually no tool wear. In this process, an electrically conductive working tool and the electrically conductive workpiece are each secured to a DC power supply, with the tool as cathode and the workpiece as anode, while a suitable electrolyte is circulated through a narrow gap maintained between the tool and workpiece. Utilizing proper operating parameters during the machining process, the workpiece is "deplated" in that area directly opposed from the cathodic tool, such that metal is selectively removed from the workpiece surface to form a negative, conjugate image of the tool face, but with some degree of anticipated variation. That is to say, because of process variables, such as electrolyte flow patterns and variables in conductivity across the gap, the electrochemically machined face, particularly in complex surface configurations, will normally be somewhat different from the face on the electrode tool. Since the result is not always predictable, a trial and error approach is often utilized to develop a tool shape as necessary to achieve the desired machined shape.
ECM has a relatively high metal removal rate, e.g. one cubic inch of metal can be removed per minute at 10,000 amperes at 1000 amperes per square inch current density. Generally, the current density is the primary factor in establishing removal rate and smoothness of the machined surface, with higher current densities creating higher removal rates and better surface finishes. In machining to any significant depth, either the tool or the workpiece must be made to progressively move toward the other for the purpose of maintaining a constant interface gap as necessary for optimum operation.
The metal atoms removed from the anodic workpiece surface by the electrochemical action normally combine with a hydroxyl radical in the electrolyte, thereby freeing hydrogen ions, both of which are carried away by the circulating electrolyte to thereby maintain high removal rates and reasonably close machining tolerances. At the cathodic tool, the electrons flowing from the anode combine with hydrogen ions forming hydrogen gas, which is also carried away by the circulated electrolyte.
There is considerable heat evolved in electrochemical machining in direct proportion to the current density. It is essential therefore, particularly at high removal rates, that a constant and uniform electrolyte flow be maintained and assured through the gap to prevent boiling of the electrolyte. Indeed, any pockets of electrolyte which do not flow readily may cause the electrolyte to boil in those locations creating vapor bubbles which will cause arcing and could virtually ruin the intended results. In order to keep the electrolyte machining characteristics constant, the electrolyte temperature, pH value and chemical concentration must be regulated during operation of the process. Accordingly, the machining apparatus is usually provided with an electrolyte treating system to adequately cool the electrolyte, vent the hydrogen, remove the metal hydroxide sludge therefrom, and if necessary, readjust the electrolyte pH.
The most common electrolyte used in ECM is an aqueous salt solution, normally, sodium chloride or sodium nitrate. Sodium chloride has been an ideal electrolyte because its chemical composition is not depleted by the ECM process since neither the sodium nor chlorine ions enter into the occurring reactions. In addition, sodium chloride has good electrical conductivity, is readily available, inexpensive, nontoxic and has a low tendency to passivate the workpiece. Other electrolytes such as sodium nitrate, for example, have a number of desirable characteristics, such as a low factor for causing stray etching. Many of such other electrolytes, however, can have a tendency to passivate the workpiece surface. "Passivation" of the workpiece surface is a condition whereby the electrolyte forms a nonconductive complex oxide film on the workpiece surface, which, by virtue of its nonconductivity, will electrically insulate the workpiece surface to prevent the desired electrochemical metal removal from progressing. For conventional ECM processes, therefore, it is essential that the electrolyte not be one that will passivate the workpiece surface to an extent sufficient to interfere with the desired machining operation.
Even though the above brief description is rather simplistic in describing the basic ECM process, space does not permit even a brief description of all the applications to which ECM has been applied with some degree of variation to the above description. Examples of such variations include: electrochemical deburring, electrochemical grinding, electrochemical discharge grinding, electrochemical honing, electrochemical polishing and others.
Of all the above noted modifications to conventional electrochemical machining, electrochemical grinding is the most pertinent to this inventive process, and involves metal removal by both an electrochemical action and a physical grinding action. In electrochemical grinding, a unique rotating grinding wheel is utilized which has a conductive interior body with nonconductive abrasive particles on the grinding surface protruding from the conductive body. Accordingly, the rotating grinding wheel not only functions to remove metal by a conventional grinding action of the protruding abrasive particles, but the conductive interior body further serves as the cathode in the electrochemical metal removing action. The nonconductive abrasive particles extending from the conductive body portion further serve to space the conductive body of the wheel from the conductive surface of the workpiece, and thereby maintain a constant gap between the tool and the workpiece as necessary to maintain an optimum electrochemical deplating action. In some applications, the grinding wheel is also of sufficient porosity that the electrolyte, necessary for the electrochemical reaction, is conveyed by centrifugal force to the grinding surface from an annular chamber at the axis of the grinding wheel. In other applications, particularly those where the physical grinding is affected by the side surface of the grinding wheel, the electrolyte is passed through an external conduit and directed to the gap between the wheel's conductive body and the workpiece. In either situation, the electrolyte is continuously drawn through the gap by the rotating particulate surface of the grinding wheel so that overheating of the electrolyte is never a problem.
The benefits derived from electrochemical grinding stem primarily from the fact that it utilizes the combined effect of ECM and conventional physical grinding with the added advantage that by merely biasing the grinding wheel against the workpiece surface, the proper gap can be maintained.
In typical applications of electrochemical grinding, a majority of the metal removal, i.e. 70 percent or more, is removed by the electrochemical reaction, while the smaller balance is removed physically by the abrasive action of the grinding wheel. Accordingly, wheel pressure on the workpiece surface is usually low so that physical wear is minimized and the need for wheel dressing is also minimized. The fact that the grinding wheel will remove surface films during the grinding operation, adds another advantage in permitting a greater choice of electrolytes. That is to say, because the mechanical grinding action of the grinding wheel will prevent the formation of surface films on the workpiece surface during the grinding operation, there is no need to be concerned about passivation of the electrolyte, and hence electrolytes can be utilized even if they would otherwise have a propensity to passivate the workpiece.
While all of the above noted ECM process variations, including electrochemical grinding, have been successful in advancing the ECM art, all have their advantages, disadvantages and limitations. Electrochemical grinding, for example, is utilized solely in face, plunge, cone or contour grinding as it is only capable of grinding surfaces that can be generated by wheel rotation, just as in conventional grinding. Because of stray etching and undercutting, it is only with great difficulty that ECM in any form is capable of machining complex three-dimensional configurations with any significant degree of precision or resolution of intricate detail.
Orbital abrading, on the other hand, is another non-traditional machining process which is entirely different and distinct from ECM. Orbital abrading involves the physical abrasion of a workpiece by a tool having an abrasive surface. Unlike conventional grinding, orbital abrading does not utilize a rotating grinding wheel, but rather brings the tool and workpiece together, whereby at least one of which is in orbital motion with respect to the other. In this process, the working tool is usually formed of a comparatively much harder material than the workpiece, and typically has a three-dimensional configuration in its working face. By orbiting either the tool or the workpiece, or both, while the tool and workpiece are in contact and biased against each other, and using a very small radius of orbit, the complementary negative configuration of the tool is worked into the workpiece. Unlike conventional grinding techniques, orbital abrading utilizes a very small relative movement, having a radius of orbit, typically from 0.020 to 0.100-inch at a typical rate, of 1200 oscillations per minute. Because of the orbital motion between the tool and the workpiece, the resulting machined configuration in the workpiece cannot be of identical size to that of the tool. However, because of the very small orbital displacement of the two pieces during working, the difference in size is small, and further permits the production of rather detailed and intricate ground configurations with a high degree of resolution, in either two or three dimensional form.
While orbital abrading is a well accepted non-traditional machining process based solely on its own merits, there are other known machining processes wherein orbital abrading, or at least orbital motion between the tool and workpiece, has been combined with other machining techniques. For example, U.S. Pat. No. 3,593,410, issued to Taylor, teaches a machining process which utilizes a vibratory motion between an abrasive tool and workpiece wherein the interface is submerged in a solution which will chemically alter the workpiece surface to facilitate the mechanical abrasion. U.S. Pat. No. 3,663,786, issued to O'Connor, teaches an electrical discharge machine tool which provides an abrasive relative motion between the electrode and workpiece, primarily for the purpose of machining graphite electrical discharge machining electrodes. Of more relevance to this invention, U.S. Pat. No. 3,564,190, issued to Kandajan et al. teaches a number of machining processes including an electrochemical machining process wherein a relative motion, orbital or otherwise, is imparted between the tool and workpiece, so that the workpiece is machined by the combined activity of the two processes. This reference does not, however, teach or suggest the unique features of this invention as essential to effect a greater degree of precision, but rather, relies merely on the combined metal removal techniques to speed the machining process.